U.S. Pat. No. 4,796,150, which issued Jan. 3, 1989 to Dickey et al, discloses a mechanical-electrical device for protecting telecommunication equipment from power surges which may occur on the tip and/or ring conductors of attached transmission lines. The device disclosed by Dickey et al offers protection against surges in both tip and ring conductors by utilizing a plurality of semiconductor devices. Six of the semiconductor devices are diodes which function as rectifiers, permitting the passage of current in only one direction. These diodes are referred to as "steering" diodes for that reason. A seventh device is a chip containing both a Zener diode and a Silicon Controlled Rectifier (SCR) and is responsive to surges. This seventh device cooperates with the six steering diodes so that surges on the tip and/or ring conductors, regardless of polarity, are conducted to ground.
Dickey et al also discloses a packaging arrangement which includes a non-conductive housing with several mechanical parts. Openings are formed in the housing to receive the semiconductors and the metallic plates associated with the semiconductors. The metallic plates act as electrical connectors and heat dissipaters. The housing disclosed by Dickey et al has a complex structure, and the semiconductors and metallic plates must be mounted manually, one at a time, which greatly adds to the assembly time and overall cost of the protection device.
Another arrangement which offers protection against surges and which includes a plurality of semi-conductor devices is disclosed by Bulley in U.S. Pat. No. 4,408,248. The arrangement disclosed by Bulley offers protection against surges in both tip and ring conductors by utilizing six steering diodes and a seventh device in the form of a Silicon Controlled Rectifier (SCR) which is responsive to voltage and current sensing circuits.
Copending Patent Application Serial No. 07/917,778 filed Jul. 21, 1992 in the name of Muni Mitchell, Robert Fried, Lou Schilling, and Willem Einthoven entitled "Surge Protector Circuit Module and Method for Fabricating Same" (which is owned by the assignee hereof) teaches a unitary circuit module adapted to be installed as a single unit in a telecommunication protection device, and which overcomes the problems associated with the Dickey et al and Bulley type devices. It employs a unique lead frame permitting the modules to be fabricated using mass production techniques. As a result, the cost of the circuit itself is reduced, as is the cost of the construction and assembly of the surge protection device.
In the invention described in the above noted patent application, sets of steering diodes and an ungated thyristor are simultaneously formed into surge protection circuit subassemblies around spaced portions of an extended lead frame. The frame includes three spaced parallel leads for each module. The leads are connected along their exposed ends by a bridge. During assembly, the sets of semiconductor components are arranged along the lead frame in a soldering fixture. By heating, each lead is soldered between a pair of steering diodes, and the steering diodes on each side of the lead frame are soldered to different conductive plates. The thyristor is also soldered between the conductive plates. The components are arranged in a "T" formation. The conductive plates are correspondingly "T" shaped.
The above circuit module employs a single, unidirectional voltage limiting thyristor chip. It is possible to use a single unidirectional thyristor because the steering diodes direct the surge current through the thyristor in the same direction, no matter which two leads or polarities are involved. Use of a single unidirectional thyristor with six steering diodes is a significant advantage over a device with either two or three bidirectional thyristors.
Conventional telecommunication surge protectors which contain two or three thyristors are commonly provided in a 3-lead "transistor outline" (TO) package. This size package is required for certain applications. However, neither the two thyristor nor the three thyristor structure is truly balanced. That is, because surges across different lead combinations do not pass through the same voltage limiting device (thyristor), the voltage limiting between any two leads is not necessarily equal. This problem can be reduced by employing "matched" devices. However, using matched devices on the chip to achieve balance is prohibitively expensive, and no two devices are exactly alike.
The use of a single, unidirectional thyristor with six steering diodes, as set forth in the above patent application Ser. No. 07/917,778, eliminates the balancing problem in a cost effective manner. However, the subassembly taught in this application, for example, cannot be packaged to fit into "standard" transistor outline package because of the specific arrangement of components and of the shape of the conductive plates between which the components are situated.
In the present application, all references to "standard" 3-lead transistor outline packages relate to JEDEC registered TO-220 and TO-263 outlines and to the "A" package modified TO-220 (isolated) SIDACTOR.RTM. distributed by Teccor Electronics, Inc. of Irving, Tex.